1. Field of the Invention
The present invention relates to an optical fiber for optical wiring in a device, and, more particularly to an optical fiber that can be bent with a small curvature radius, and an optical fiber ribbon and optical interconnection system using the optical fiber.
2. Description of the Related Art
There are two methods for signal transmission in a device, i.e., electrical transmission and optical interconnection. With a recent increase in clock frequency of a CPU, the electric transmission has a problem of crosstalk due to high-density wiring, thereby requiring application of a technique such as waveform shaping. Consequently, it has turned out that when applied as a signal transmission method in a device, the electrical transmission has a transmission limit with a transmission distance of about 1 meter and a transmission rate of about 10 Gbps.
Meanwhile, the optical interconnection allows a broader band transmission than the electrical transmission, and can establish a signal transmission system using optical components that are smaller and consume lower power. Thus, the optical interconnection draws attention as a technique for signal transmission in a device, replacing the electrical transmission.
One optical transmission means using optical fibers according to the optical interconnection attracts attention. It is desirable that all optical components used in a device be accommodated in a small space as much as possible. Therefore, optical fibers that are capable of flexible wiring and produce a reduced splice loss in connection of optical components, such as an optical fiber and an optical fiber, and an optical fiber and an optical source, are desirable to be used in the optical interconnection.
As an optical source, a Vertical-Cavity Surface-Emitting Laser (hereinafter, VCSEL) that operates under direct modulation from 2.5 Gbps to 10 Gbps draws attention as an uncooled source for Ethernet (registered trademark) and fiber channel of an access system. The VCSEL has the following characteristics as compared to an edge emitting laser such as a distributed feedback (DFB) laser.
(a) Laser beams are emitted vertically from a substrate surface.
(b) Multi-channel arraying is easy.
(c) It can operate with a low threshold and low power consumption.
(d) An emitting surface has high reflectance and is resistant to reflected light (isolator free).
(e) Emitted beams have a circular shape and a coupling ratio with fibers is high (lens free).
Because components such as the isolator and the lens can be eliminated, the VCSEL is a device that can reduce costs as a module. A VCSEL at a waveband of 850 nanometers using a GaAs/AlGaAs quantum and the like as an active layer has become increasingly common as a de facto standard of a laser element for a short-range communication. An example of typical optical fibers to be used is a silica graded-index optical fiber, which is one type of multimode fibers (hereinafter, MMF).
The MMF has a core diameter about ten times that of a single mode fiber (hereinafter, SMF), and has a large number of openings. Therefore, the MMF does not require a high accuracy in connecting optical components such as an optical fiber and an optical fiber, and an optical fiber and an optical source, and allows easier connection.
Recently, application of the SMF having a lower loss and a wider band than that of the MMF has begun being discussed to realize a higher-speed transmission. The VCSEL having an oscillation wavelength of 1.3 micrometers (1300±50 nanometers), which is a low loss band of a silica optical fiber, draws attention as an optical source to be used, and is actively studied and developed.
However, a standard SMF defined by ITU-T (International Telecommunication Unit Telecommunication Standard Sector) G.652 cannot address occurrence of a large bending loss when accommodated in a device.
An optical fiber that has a trench refractive index profile in a cladding portion of the standard SMF with a lower index portion than the cladding is reported as an SMF with a reduced bending loss, which is an optical fiber suitably used for FTTH (Fiber To The Home) (for example, in Masataka IKEDA, Shoichiro MATSUO, Kuniharu HIMENO, “Low bending loss optical fiber with reduced splice loss”, Technical report of the Institute of Electronics, Information and Communication Engineers, OCS2003-43, OFT2003-25 (2003-8)). However, such an SMF is insufficient for the optical interconnection system.
As described above, realization of an optical fiber that has a reduced bending loss and splice loss, provides high-speed optical transmission, and is suitable for easily establishing an optical interconnection system is demanded to be used for the optical interconnection system.
The standard SMF causes a large bending loss, and is not applicable. Thus, reduction of the bending loss in the optical fiber is required. However, even the SMFs having reduced bending losses, including the optical fiber described in the above literature, which are suitably used for the FTTH are insufficient to be used in the optical interconnection system.